1. Field of the Invention
The invention relates in general to porous composite media, and, in particular, to filters which operate under extreme conditions of thermal and pressure shock to perform multiple functions, and to their method of preparation.
2. Description of the Prior Art
Certain filter applications require that hot gases from what is in effect an explosion be reliably filtered to remove particulate material, control the flow of hot gas, and moderate the temperature and pressure of the gas. One such application is that of automobile safety crash bags. Such crash bags (airbags) typically consist of a solid propellant gas generator, a filter, a housing and the bag. At the onset of the crash, a sensor activates an ignitor which "lights off" the solid propellant charge located in the center. Hot, high pressure gas including some entrained particulate material is generated, passed through a filter and into the bag. Total operating time is typically 50-100 milliseconds. The initial instantaneous pressure pulse may be as high as 1500 to 4,000 pounds per square inch, and the initial instantaneous temperature of the gas may be as high as 1,500 to 3,000 degrees Fahrenheit. Large quantities of particulate material with a wide particle size distribution are generated. The filter should in essence accept the gas generated in an explosion and modify it so that it will safely and rapidly inflate but not rupture an airbag. The modification of the explosion's gaseous byproducts should include the removal of particulate matter which might puncture the airbag and significantly contaminate the breathable air in the vehicle, cooling the byproducts so that they will not melt the bag or injure the user, buffering the initial pressure pulse, limiting the flow rate so that the gas will not rip the bag, and distributing the gas uniformly throughout the volume so that it will not overload and rip any part of the bag during inflation. Previous expedients were generally deficient in achieving these objectives.
An excessively hot gas stream may burn the user. Thus, the gas should have a temperature of no greater than approximately 600 degrees Fahrenheit when it contacts and commences to expand the airbag. As the gas enters the collapsed airbag and begins to expand it the inertia of the airbag walls requires that the rate at which the gas flows should be limited. Also, a blast of high pressure gas might injure the user. Thus, the pressure pulse must be damped by the filter to slightly above atmospheric pressure, for example, approximately 15 pounds per square inch. The airbag filter should withstand pressure pulses as high as 3,000 or even 4,000 pounds per square inch and temperature pulses as high as 3,000 or at least 2,732 degrees Fahrenheit. The efficiency of the airbag filter should be such that no more than, for example, approximately 2 grams of particulate material passes through the filter into the airbag. In order to accomplish these ends the filter must permit the hot, high velocity, inflation gases to pass uniformly through the filter media without channelling. To accomplish these objectives the filter should be structurally and thermally stable, that is it should withstand the highest anticipated pressure and thermal pulses without significant degradation. Significant degradation occurs when the filter is physically damaged to the extent that it fails to achieve one or more of its objectives. The filter media should be machinable in a manufacturing process to a tolerance of plus or minus five thousandths of an inch. It should also be capable of withstanding a press fit. Previous expedients were generally deficient in meeting many of these requirements, particularly in being structurally and thermally stable.
The key component is the filter which performs several functions. The filter must have sufficient strength, toughness and temperature resistance to survive the initial instantaneous pressure and thermal pulses, provide a predetermined flow resistance to permit the filling of the airbag without subjecting it to the initial pressure pulse, provide sufficient particulate capture efficiency, provide some thermal capacity so that hot gas is not expelled into the airbag and produce reliable, predictable, uniform flow through the filter. Previously proposed expedients were generally deficient in meeting one or more of these environmental requirements.
Previously proposed airbag filters were typically composed of an inner and outer wire or perforated cage, which provided the required mechanical strength and held the filter media in place. Typically, a ceramic fiber felt was used as the kilter media. Woven metal mesh was also proposed previously. The ceramic fiber felt had essentially no strength and had to be supported in the wire cages. Due to the inherent non-uniformity and limited strength of the previous filters, the gasses often channeled unpredictably through the filter media, resulting in non-uniform, non-repeatable filtration and flow. This channeling, inter alia, permitted hot gas and a large amount of large sized particulate matter to be expelled into the bag. This often resulted in burning the automobile occupant, over or under inflating the airbag, or the physical failure and/or explosion of the device.
Other filter materials have been proposed with similarly poor results. Ceramic filters generally have very poor thermal shock resistance, resulting in cracking and filtration failure. Metal filters generally will not survive the temperature and pressure excursions and still provide the necessary uniform filtering. Further, previously proposed filters generally did not have the pore size, shape, strength and uniformity required to uniformly distribute the gas, optimize the gas flow and trap particles efficiently.
These and other difficulties of the prior art have been overcome according to the present invention.